The Sapat Complex in Northern Pakistan contains remnants of the northern Neo-Tethys Ocean, presently exposed along the Indus Suture Zone. The mantle peridotites of the Sapat Complex include harzburgites, dunites, and subordinate lherzolites. Harzburgites are the dominant peridotite variety over dunites. The dunites are hosted by harzburgites and occur exclusively as ‘envelopes’ surrounding chromitite pods. The podiform chromitites show disseminated, banded, and massive textures. Chromitites exhibit variable Cr# [Cr/(Cr + Al)] and Mg# [Mg/(Mg + Fe²⁺)], which range from 0.76 to 0.77, and from 0.64 to 0.66, respectively, while TiO₂ contents are <0.2 wt.%. These features perhaps reflect crystallization of the chromian spinel from a boninitic magma. Similarly, chromian spinel in peridotites manifest a wide range of Cr# and Mg#, from 0.49 to 0.83 and 0.41 to 0.57, respectively, and are characterized by very low TiO₂ values, averaging at 0.1 wt.%. Chromian spinel of chromitites and peridotites of the Sapat Complex have also very low Fe³⁺# (<0.01), which indicate their crystallization under low oxygen fugacities. The platinum group elements (PGE) distributions show high (Os + Ir + Ru)/(Rh + Pt + Pd), very low Pd/Ir values, and are defined by a prominently fractionated chondritic normalized PGE pattern, hence, this deposit is a typical example of an ophiolitic chromitite. The studied peridotites are highly depleted in PGE compared to chondritic values. The Pd_N/Ir_N values, averaging to 1.5 in dunites are unfractionated, while PGE spidergrams of harzburgites and lherzolites depict minor positive slopes, a minor positive Ru anomaly, and have average Pd_N/Ir_N values of 2.3 and 2.4, respectively. Furthermore, the harzburgites, dunites, and lherzolites display generally flat chondritic and primitive mantle normalized PGE patterns, and therefore, are nearly identical to highly depleted mantle peridotites. The mineralogical and PGE geochemical imprints of Sapat Complex chromitites and peridotites establish a strong affinity to supra-subduction zone ophiolites. Moreover, calculated parental melts of the chromitites and various geochemical discrimination diagrams elucidate that the chromitites were derived from boninitic magma produced by melting of depleted mantle peridotites in an oceanic arc, characterized by low oxygen fugacity, similar to a supra-subduction zone tectonic setting. This research highlights the use of mineralogy and geochemical compositions of chromitites and peridotites to reveal deep magmatic processes in a supra-subduction zone environment. Graphical Abstract

The Kargah Cu-Pb polymetallic deposit is a newly discovered ore deposit from the Gilgit-Baltistan region, located in the Kohistan Island Arc, northern Pakistan. However, this area is poorly researched on the ore genesis, and its origin and the evolution of its magmatic-hydrothermal system remain unclear. Three stages of mineralization were identified, including quartz-pyrite, quartz-sulfide, and carbonate representing early, middle, and late stages, respectively. The major ore minerals are pyrite, chalcopyrite, galena, and zincian tetrahedrite with minor native silver, and native gold mainly distributed in pyrite. Here, we present a systematic study on ore geology, hydrothermal alterations, trace element composition of pyrite, fluid inclusions, and isotopes (S and Pb) characteristics to gain insights into the nature of the ore-forming fluids, types of unknown deposits, and hydrothermal fluid evolution. The high Co/Ni ratio (1.3–16.4) and Co content (average 1201 ppm), the low Mo/Ni ratio (0.43–0.94) and Mo contents (average 108 ppm) of both Py-I and Py-II suggest a mafic source for the mineralization. The Au-Ni plots, Co-As-Ni correlation, and the δ³⁴S values range from −2.8 to 6.4‰ (average of 3.4‰) indicating the affiliation of the mineralization with a mantle-derived magmatic-hydrothermal provenance. The Pb isotope data showing the narrow variations in ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb values suggest a single lead source from crustal-derived materials. The microthermometry data suggest that the dominant mechanisms are fluid boiling and mixing for mineral precipitation at temperatures ranging between 155 and 555 °C, and represent an intrusion-related magmatic-hydrothermal environment for the Kargah Cu-Pb polymetallic deposit.